A road safari from Arusha to the central Serengeti is a full day, and the return is another. On a seven-day trip that is two days of your holiday spent looking at the back of a seat.
A fly-in safari removes them. You fly from Arusha to a Serengeti airstrip in about an hour, and a vehicle and guide from your camp meet the aircraft — often with a game drive on the way to camp, because the strip is inside the park. At the end you fly out, sometimes straight to Zanzibar without going back through Arusha at all.
It costs more than driving, and it means a different guide and vehicle at each camp rather than one team throughout. In exchange you get more days in the parks, no long transfers, and an aerial view of the Rift Valley and the herds that is worth the ticket on its own. For anyone with under ten days, or with a bad back, we recommend it strongly.